Voltage-dependent sodium channels are responsible of the rising phase of the action potential in excitable cells. These integral membrane proteins are composed of a pore-forming α-subunit, and one or more auxiliary β subunits. Mutation p.Asp25Asn (D25N; c.73G > A) of the β1 subunit, coded by the gene SCN1B, has been reported in a patient with generalized epilepsy with febrile seizure plus type 1 (GEFS+). In human embryonic kidney 293 (HEK) cells, the heterologous coexpression of D25N-β1 subunit with Nav1.2, Nav1.4, and Nav1.5 α subunits, representative of brain, skeletal muscle, and heart voltage gated sodium channels, determines a reduced sodium channel functional expression and a negative shift of the activation and inactivation steady state curves. The D25N mutation of the β1 subunit causes a maturation (glycosylation) defect of the protein, leading to a reduced targeting to the plasma membrane. Also the β1-dependent gating properties of the sodium channels are abolished by the mutation, suggesting that D25N is no more able to interact with the α subunit. Our work underscores the role played by the β1 subunit, highlighting how a defective interaction between the sodium channel constituents could lead to a disabling pathological condition, and opens the possibility to design a mutation-specific GEFS+ treatment based on protein maturation.